1. Background of the Invention
The invention relates to semiconductor wafer processing systems and, more particularly, to a system for failure prediction of electrostatic chucks used in semiconductor processing.
2. Description of the Background Art
Substrate support chucks are widely used to support and retain substrates (e.g., semiconductor wafers) within semiconductor processing systems. A particular type of substrate support chuck used in high temperature physical vapor deposition (PVD) systems is an electrostatic chuck or E-chuck. These chucks are used to retain semiconductor wafers, or other substrates, in a stationary position in a process chamber during processing. Such electrostatic chucks contain one or more electrodes embedded within a unitary chuck body. The chuck body is, for example, fabricated of aluminum nitride or boron nitride or alumina doped with a metal oxide such as titanium oxide or chromium oxide or some other ceramic material with similar electrical and thermal properties.
In use, a wafer rests against the support surface of the chuck body as a chucking voltage is applied to the electrodes. Because of the conductive nature of the ceramic material at high temperatures, the wafer is primarily retained against the support surface of the ceramic chuck by the Johnsen-Rahbek effect. The Johnsen-Rahbek effect establishes a small but highly effective electric current between the upper surface of the chuck and the substrate being retained. As such, charges migrate to the support surface of the chuck and the underside of the substrate resulting in the substrate being retained against the support surface of the chuck by a chucking force which is much greater than that generated by a purely Coulombic effect between the wafer and the electrodes. Johnsen-Rahbek chucks are disclosed in U.S. Pat. Nos. 5,117,121, issued May 26, 1992 and 5,463,526 issued Oct. 31, 1995.
During wafer processing, the chuck can be subject to a wide range of temperatures, typically in the range of 20.degree. to 150.degree. C. and possibly as high as 300.degree. to 500.degree. C. for some types of processes. At room temperature (approximately 20.degree. C.), the resistivity of the chuck is on the order of 10.sup.14 ohm-cm and decreases to approximately 10.sup.13 ohm-cm at temperatures of around 150.degree. C. This decrease in resistivity promotes a satisfactory chucking force via the Johnsen-Rahbek effect. At temperatures above about 350.degree. C., a conductive film can form on the chucking surface as a result of exposing the chuck to carbon-containing compounds in the chamber atmosphere, e.g., seal outgassing, residual air, and the like. Such a conductive film causes a leakage current when a chucking voltage is applied to the chucking electrodes that enables the surface charge to dissipate into metallic components contacting the chuck or conduct into the wafer. At about 550.degree. C., the leakage current can be as high as 6 milliamps, which is sufficient to cause arcing between the wafer and the chuck. If the leakage current is 10 to 15 milliamps, few charges accumulate on the chuck surface and the chucking force is drastically reduced or lost completely. If a suitable chucking force is not developed by the chuck, wafer processing can be hindered to the point that yields are reduced or entire batches of wafers are ruined. The conductive material can be cleaned from the surface of the chuck by plasma cleaning procedures. Plasma cleaning techniques that can be used to clean electrostatic chucks are disclosed in U.S. Pat. Nos. 5,221,450, issued Jun. 22, 1993 and 5,507,874, issued Apr. 16, 1996.
Wafer process chambers are equipped with system controllers which provide the necessary voltages, currents, and signals for chucking, heating, initiating cleaning sequences or controlling process sequences. Chucking voltages (in the range of 0-1000 V) are supplied by a high voltage power supply responsive to a control signal from the system controller. However, in prior art process controllers, the cleaning sequence must be initiated at the discretion of an operator. Often, the operator is unaware of the build-up of the conductive film until the leakage current has become so large as to inhibit the chuck from developing a suitable chucking force (i.e., chuck failure) by which time a batch of wafers may already be ruined.
Therefore, a need exists for a method and apparatus that informs the operator, in advance, that an electrostatic chuck is about to fail and that it is time to clean the chuck.